This invention relates to liquefied gas containers and more particularly to such containers having two walls that are sealed together and evacuated between the walls for low heat transfer and the liquid is drawn from the container through a heat exchanger where it is turned to a gas and fed on demand as a gas to a user. The advantage of such containers over high pressure gas containers for the same gas is that the gas is stored as a low (cryogenic) temperature liquid at relatively low pressure and the liquid volume stored is substantially less than the high gas pressure volume stored in conventional high pressure containers.
Heretofore, containers for low temperature liquefied gas have included an inner stainless steel shell that contains the low temperature liquified gas and an outer stainless steel shell that encloses the inner shell and is sealed to it. The space in between the shells is evacuated and the connections and support structures between the inner and outer shell are held to a minimum, because each such connection and structure is a conduit for heat. It has been the practice to provide a passage or channel at the top of the container from the outside to inside the inner shell through a thin wall stainless steel tube, sometimes called a neck, that is sealed to an opening in the inner shell and projects through and is sealed to an opening in the outer shell. The thin wall tube provides access to the inner shell from outside of the container and is thin walled so that it will be a minimum conductor of heat from the outer shell to the inner shell.
One of the problems with such prior containers is that the top end of the inner shell is supported within the outer shell only by the thin wall tube. This suspension with no support at the bottom of the container acts like a pendulum. When the container is tilted, the thin wall tube (called the neck) will often break. If the inner shell is also supported by the outer shell at the bottom of the container, the bottom support does not penetrate the shells and so does not have to make vacuum seals with them and so can be very sturdy and made of low thermal conductivity material. In that case, the lateral forces on the thin wall tube at the top when the container is tipped are worse, because the inner shell cannot swing as a pendulum against the outer shell and so the thin wall tube cannot bend to release the lateral forces and will rupture sooner than without the bottom support.
Another feature of prior containers is that the low temperature liquid, such as liquid nitrogen, is loaded into the container inner shell through a thin wall stainless steel input tube that extends from outside the container through the neck to the inner shell to substantially the bottom thereof. Low temperature liquid is drawn from the container by another stainless steel tube, the output tube, that enters the inner shell at the bottom from the evacuated space between the shells and immediately attaches to a copper tube that extends upward in the evacuated space between the shells and is attached by soldering to the inside of the outer shell. This copper tube often circles around the inner shell several times like a coil to provide a large surface area for conducting heat from the outer shell into the liquid to vaporize it as it is drawn through the copper tube. At the top of the container the copper tube connects to a stainless steel tube that penetrates and is welded to the outer shell and then to a gas output valve outside the container.
Another problem with such prior containers is that any leaks in the vacuum space between the shells will ruin the performance of the container. Any metal parts (for example support structures and the input and output tubes) and gas flow inside the output tubing running between the shells adds to the heat leak significantly.
As a consequence of these problems with prior containers and the high heat leakage, the user had to use many container loads each week to justify the cost of using the container. As mentioned, the copper vaporizer coil is soldered to the inside of the stainless steel outer shell. In order to solder these metals, corrosive flux must be used which leaves a residue that later produces gases in vacuum and so reduces the effectiveness of the vacuum space. Furthermore, because of the high conductivity of copper tubing, it is impractical to run copper tubing from the outer shell back to the inner shell of the container through the vacuum space and so several transitions are required from stainless steel to copper tubing inside the vacuum space with a soldered joint for each transition.